This application claims priority of Japanese patent application number 2000-083997, filed Mar. 24, 2000.
This invention relates to an optical TDM multiplexer to multiplex a plurality of signals in the optical stage in the time domain and an optical TDM demultiplexer to demultiplex time-division-multiplexed (TDM) signals in the optical stage.
This invention also relates to a WDM/TDM converter to convert an optical WDM signal into an optical TDM signal and a TDM/WDM converter to convert an optical TDM signal into an optical WDM signal.
In optical fiber communication, a transmission rate has increased from 10 Gb/s to 20 Gb/s and 40 Gb/s. To cope with such increase of the transmission rate, it requires a device to time-division-multiplex a plurality of signals of 10 Gb/s, and conversely a device to demultiplex a time-division-multiplexed signal into individual signals. It is possible to consider the multiplexing/demultiplexing in both electric and optical stages. However, since the electric performance is limited, the multiplexing/demultiplexing in the optical stage is more desirable.
For instance, an optical time division multiplexing system is well known that multiplexes optical short pulses with a sufficiently narrow optical pulse width in the optical time domain. To demultiplex a TDM signal in the optical stage, an optical circuit is necessary to logically perform AND operation of a high-speed optical signal and a low-speed short pulse. Representative examples of such AND circuit are a nonlinear loop mirror and an element using four-wave mixing of an optical fiber.
With regard to the multiplexing, there are problems as follows. That is, to multiplex signals of 10 Gb/s in the electric domain in the time domain, a high-speed electronic circuit is required. In addition, an optical modulator to convert an electric signal into an optical signal generally needs a large driving voltage proportional to an increase of the operation rate while an output voltage of the electronic circuit decreases as the operation rate becomes faster. Accordingly, it is impossible to obtain desired characteristics of such as an extinction ratio using a conventional system that drives an optical modulator with a multiplexed signal obtained through the time-division-multiplexing in the electric domain.
In addition, in a conventional optical time division multiplexing system to multiplex optical short pulses with sufficiently narrow optical pulse width in the optical time domain, there are strict requirements such as the extinction ratio of each low-speed signal is sufficiently high and the pulse width of each optical pulse is sufficiently narrow in order to avoid noise caused by interference between adjacent pulses. For instance, when four low-speed signals at 10 Gb/s are multiplexed, the pulse width of each 10 Gb/s signal has to be no more than ⅕ to xc2xc of an objective 40 Gb/s bit time slot (25 ps), namely 5 ps to 6 ps. To realize the above condition, a large and high priced short pulse light source is required and thus the cost is greatly increased.
Meanwhile, the existing optical AND circuit cannot perform the demultiplexing stably either since it is sensitive to a polarization fluctuation and a temperature fluctuation.
It is therefore an object of the present invention to provide an optical TDM multiplexer, an optical TDM demultiplexer, a WDM/TDM converter and a TDM/WDM converter to have superior characteristics to those of prior art.
Another object of the present invention is to provide an optical TDM multiplexer, an optical TDM demultiplexer, a WDM/TDM converter and a TDM/WDM converter to operate in stable condition and to be realized low-priced.
Further object of the present invention is to provide an optical TDM multiplexer, an optical TDM demultiplexer, a WDM/TDM converter and a TDM/WDM converter to operate faster than ever.
An optical TDM multiplexer according to the invention is the apparatus to multiplex a plurality of input signals in the optical stage in the time domain, composed of a plurality of signal light sources to generate optical signals having a wavelength different from each other to carry each of the plurality of the input signals, a timing adjuster to adjust timings between the respective optical signals so that each optical signal output from the plurality of the signal light sources is disposed on a time slot different from the others in the time domain, an optical multiplexer to multiplex optical signals output from the timing adjuster in the wavelength domain, and a wavelength converter to convert each wavelength of the output light from the optical multiplexer into a predetermined wavelength.
With the above configuration, a plurality of signals at a high bit rate can be multiplexed in the time domain without using a large and expensive short pulse light source. Moreover, the stable operation can be expected.
It is possible to remove unnecessary components by disposing an optical filter to extract the light having the predetermined wavelength out of the output light from the wavelength converter.
Preferably, the wavelength converter is composed of a probe light source to generate probe light having the predetermined wavelength, a waveform superimposer applied by the output light from the probe light source and from the optical multiplexer to superimpose a signal waveform of the output light from the optical multiplexer on the probe light, and an outputter to output the probe light transmitted through the waveform superimposer. With this configuration, a wavelength of signal light as fast as 40 Gb/s is converted into a predetermined wavelength.
A WDM/TDM converter according to the invention is the apparatus to convert an optical WDM signal composed of a plurality of optical signals each having a wavelength different from the others into an optical TDM signal, composed of a timing adjuster to adjust the timings between the plurality of the optical signals so that the plurality of the optical signals each having a different wavelength from the others to compose the WDM signal are respectively disposed on a time slot different from each other in the time domain, an optical multiplexer to multiplex the respective optical signals output from the timing adjuster in the wavelength domain, and a wavelength converter to convert a wavelength of the output light from the optical multiplexer into a predetermined wavelength.
According to the above configuration, an optical WDM signal as fast as more than 10 Gb/s can be simply and inexpensively converted into an optical TDM signal. In addition, the operation is stable.
It is possible to remove unnecessary components by disposing an optical filter to extract the light having the predetermined wavelength out of the output light from the wavelength converter.
Preferably, the wavelength converter is composed of a probe light source to generate probe light having the predetermined wavelength, a waveform superimposer applied by the output light from the probe light source and from the optical multiplexer to superimpose a signal waveform of the output light from the optical multiplexer on the probe light, and an outputter to output the probe light transmitted through the waveform superimposer. According to this configuration, a wavelength of signal light as fast as 40 Gb/s can be converted into a predetermined wavelength. That is, an optical TDM signal at a bit rate of more than 40 Gb/s is easily produced.
An optical TDM demultiplexer according to the invention is the apparatus to demultiplex an optical TDM signal having n (n is an integer not less than 2) time slots into individual signals on the respective time slots, composed of a pulse light generator to generate optical pulse trains each having a wavelength different from the others at timings each belonging to a different time slot from the others, an optical multiplexer to multiplex the n optical pulses output from the pulse light generator in the wavelength domain, a waveform superimposer applied by the optical TDM signal and the output light from the optical multiplexer to superimpose a signal waveform of the optical TDM signal onto the output light from the multiplexer, and a wavelength demultiplexer to demultiplex the output light from the optical multiplexer transmitted through the waveform superimposer into respective wavelengths.
With this configuration, it is possible to demultiplex an optical TDM signal composed of optical signals as fast as 10 Gb/s or more into individual signals. In addition, it is realized with a simple and inexpensive configuration, and the operation is stable.
Preferably, the optical TDM demultiplexer is further composed of a photodetector to convert the optical signal having a predetermined wavelength output from the wavelength demultiplexer into an electric signal and a clock generator to generate a clock signal synchronized with a clock component included in the output from the photodetector, wherein the pulse light generator generates the respective optical pulse trains in synchronization with the clock output from the clock generator. With this configuration, an optical TDM signal is demultiplexed into individual signals without fail.
Preferably, the optical TDM demultiplexer is further composed of a time slot detector to detect a time slot discrimination signal out of the output from the photodetector and to adjust time slot dispositions of the respective optical pulse trains generated by the pulse light generator according to the detected result. Alternatively, the optical TDM demultiplexer is further composed of an optical delay device capable of changing delay time to delay the optical TDM signal before entering the waveform superimposer and a time slot detector to detect a time slot discrimination signal out of the output from the photodetector and to adjust the delay time of the optical delay device according to the detected result so that the optical pulse train output from the optical multiplexer and the optical TDM signal delayed by the optical delay device have predetermined phase relations. With these configurations, it is possible to determine the correspondence between a signal after TDM demultiplexing and a time slot to which the signal belongs before the demultiplexing. The latter configuration is simpler than the former.
Preferably, the optical TDM demultiplexer is further composed of a photodetector to convert the optical TDM signal into an electric signal and a clock generator to generate a clock signal at 1/n frequency synchronized with a clock component included in the output from the photodetector, wherein the pulse light generator generates the respective optical pulse trains in synchronization with the clock output from the clock generator. According to this configuration, an optical TDM signal also can be demultiplexed into individual signals without fail.
A TDM/WDM converter according to the invention is the apparatus to convert an optical TDM signal having n (n is an integer no less than 2) time slots into an optical WDM signal having n optical signals each having a different wavelength from the others, composed of a pulse light generator to generate n optical pulse trains each having a wavelength different from the others at timings each belonging to a time slot different from the others, an optical multiplexer to multiplex the n optical pulse trains output from the pulse light generator and a waveform superimposer applied by the optical TDM signal and output light from the optical multiplexer to superimpose a signal waveform of the optical TDM signal onto the output light from the multiplexer. With this configuration, optical signals on different time slots included in a high-speed optical TDM signal can be converted into optical signals each having a wavelength different from the others.
Preferably, the TDM/WDM converter is further composed of a extractor to extract signal light having a predetermined wavelength out of the output light from the optical multiplexer transmitted through the waveform superimposer, a photodetector to convert the output light from the extractor into an electric signal and a clock generator to generate a clock signal synchronized with a clock component included in the output from the photodetector. In addition, the pulse light generator generates respective optical pulse trains in synchronization with the clock output from the clock generator. According to this configuration, an optical TDM signal can be converted into an optical WDM signal without fail.
Preferably, the TDM/WDM converter is further composed of a time slot detector to detect a time slot discrimination signal out of the output from the photodetector and to adjust time slot dispositions of the respective optical pulse trains generated by the pulse light generator according to the detected result. Alternatively, the optical TDM/WDM converter is further composed of an optical delay device capable of changing delay time to delay the optical TDM signal before entering the waveform superimposer and a time slot detector to detect a time slot discrimination signal out of the output from the photodetector and to adjust the delay time of the optical delay device according to the detected result so that the optical pulse train output from the optical multiplexer and the optical TDM signal delayed by the optical delay device have predetermined phase relations. With these configurations, it is possible to determine the correspondence between a signal after TDM demultiplexing and a time slot to which the signal belongs before the demultiplexing. The latter configuration is simpler than the former.
Preferably, the TDM/WDM converter is further composed of a photodetector to convert the optical TDM signal into an electric signal and a clock generator to generate a clock signal at a 1/n frequency synchronized with a clock component included in the output from the photodetector, wherein the pulse light generator generates the respective optical pulse trains in synchronization with the clock output from the clock generator. According to this configuration, an optical TDM signal also can be demultiplexed into individual signals without fail.